Excimer lasers are well known. An important use of excimer lasers is to provide the light source for integrated circuit lithography. The type of excimer laser currently being supplied in substantial numbers for integrated circuit lithography is the KrF laser which produces ultraviolet light at a wavelength of 248 nm. A similar excimer laser, the ArF laser, provides ultraviolet light at 193 nm. These lasers typically operate in a pulse mode at pulse rates such as 1,000 Hz. The laser beam is produced in a laser chamber containing a gain medium created by the discharge through a laser gas between two elongated electrodes of about 28 inches in length and separated by about 5/8 inch. The discharge is produced by imposing a high voltage such as about 20,000 volts across the electrodes. For the KrF laser, the laser gas is typically about 1% krypton, 0.1% fluorine and about 99% neon. For the ArF laser the gas is typically about 3 to 4% argon, 0.1% fluorine and 96 to 97% neon. In both cases in order to achieve high pulse rates of about 1,000 Hz, the gas must be circulated between the electrodes at speeds of about 500 to 1,000 inches per second.
Fluorine is the most reactive element known and it becomes even more reactive when ionized during the electric discharge. Special care must be exercised to utilize in these laser chambers materials such as nickel coated aluminum which are reasonably compatible with fluorine. Further, laser chambers are pretreated with fluorine to create passification layers on the inside of the laser chamber walls. However, even with this special care, fluorine will react with the walls and other laser components which results in a relatively regular depletion of the fluorine. The rates of depletion are dependent on many factors, but for a given laser at a particular time in its useful life, the rates of depletion depend primarily on the pulse rate and load factor if the laser is operating. If the laser is not operating, the depletion rate is substantially reduced. The rate of depletion is further reduced if the gas is not being circulated. To make up for this depletion, new fluorine is typically injected at intervals of about 1 to 3 hours. Rather than inject pure fluorine it is a typical practice to inject a mixture of 1% fluorine, 1% krypton and 98% neon. For example, in a typical 1000 Hz KrF excimer laser used for lithography, the quantity of its fluorine, krypton, neon mixture injected to compensate for the fluorine depletion varies from about 60 scc per hour when the laser is not operating and the laser gas is not being circulated to about 600 scc per hour when the laser is running continuously at 1000 Hz. The typical injection rate is about 120 scc per hour when the chamber fan is circulating the laser gas, but the laser is not firing. A typical operating mode of about 300 pulse bursts separated by a "dead time" of 0.2 second will produce an average injection rate of about 180 scc per hour.
The unit "scc" refers to "standard cubic centimeters". Other commonly used units for describing quantities of fluorine in a particular volume are percent (%) fluorine, parts per million and kilo Pascals, the latter term sometimes refers to the partial pressure of the fluorine gas mixture and sometimes to the partial pressure of fluorine alone. A 180 scc per hour injection rate of the 1% fluorine mixture would correspond to a depletion decrease in the fluorine concentration over 2 hours from about 0.11 percent to about 0.09 percent and to a decrease in the partial pressure of the fluorine neon gas mixture from about 28 kPa to about 24 kPa. The actual quantity of fluorine depleted in two hours as measured in grams of pure fluorine would be about 6.1 milligrams during the two hour period corresponding to the above 180 scc/hour injection rate (i.e., 360 scc of the 1% fluorine mixture injected during the two-hour interval) of the fluorine gas mixture.
For integrated circuit lithography a typical mode of operation requires laser pulses of constant pulse energy such as 0.010 J/pulse at about 1000 Hz which are applied to a waffer in bursts such as about 300 pulses (with a duration of about 300 milliseconds) with a dead time of a fraction of a second to a few seconds between bursts. Their mode of operation may be continuous 24 hours per day, seven days per week for several months, with scheduled down time for maintenance and repairs of, for example, 8 hours once per week or once every two weeks. Therefore, these lasers must be very reliable and substantially trouble-free.
In typical KrF and ArF excimer lasers used for lithography, high quality reproducible pulses with desired pulse energies of about 0.01 J/pulse may be obtained over a substantial range of fluorine concentration from about 0.08 percent (800 parts/million or about 24 kPa) to about 0.12 percent (1,200 parts/million). Over the normal laser operating range the discharge voltage required to produce the desired pulse energy increases as the fluorine concentration decreases (assuming other laser parameters remain approximately constant). FIG. 1 shows the typical relationship between discharge voltage and fluorine concentration for constant pulse energy of 0.014J and 0.010J.
Prior art techniques typically utilize the relationship between discharge voltage and fluorine concentration to maintain constant pulse energy despite the continuous depletion of fluorine. The discharge voltage of prior art excimer lasers can be changed very quickly and accurately and can be controlled with electronic feedback controls set to monitor pulse energy and maintain it constant. Accurate precise control of fluorine concentration in the past has proven difficult. Therefore, in typical prior art KrF and ArF laser systems, the fluorine concentration is allowed to decrease for periods of about 1 to 4 or 5 hours while the discharge voltage is adjusted by a feedback control system to maintain constant pulse energy output. Periodically at intervals of about 1 to a few hours, fluorine is injected during short injection periods of a few seconds to a few minutes. Thus, in normal operations fluorine concentration gradually decreases from (for example) about 0.11 percent to about 0.09 percent over a period of about 1 to a few hours while the discharge voltage is increased over the same period from for example about 600 volts to about 640 volts. The injection of fluorine at the end of the 1 to a few hour period (when the voltage has drifted up to about 640 volts) brings the fluorine concentration back to about 0.11 percent and the feedback control automatically reduces the voltage back to 600 volts. This basic process is typically repeated for several days. Since contamination gradually builds up in the laser gas over a period of several days, it is usually desirable to replace substantially all of the gas in the laser with new laser gas at intervals of about 5-10 days. FIG. 2 described the prior art fluorine injection technique discussed above.
The above-described prior art technique is effectively used today to provide long term reliable operation of these excimer lasers in a manufacturing environment. However, several laser parameters, such as bandwidth, beam profile and wavelength, are adversely affected by the substantial swings in the discharge voltage and fluorine concentration.
A substantial number of techniques have been proposed and patented for measuring and controlling the fluorine concentration in excimer lasers to within more narrow limits than those provided under the above described prior art technique. These techniques have generally not been commercially pursued.
What is needed is a better system and method for dealing with the issue of fluorine depletion in KrF and ArF excimer lasers.